Crohn's disease (CD) is a chronic inflammatory disorder of the gastrointestinal tract. The cause remains poorly understood. Current treatments utilize immunosuppressive medications and are often complicated by serious infections. We approached this disease from a different perspective based on observations made in patients with genetic syndromes involving impaired innate immunity that also develop Crohn's disease. These patients showed clinical responses to granulocyte-macrophage colony stimulating factor (GM-CSF), an agent that stimulates innate immunity. Phase I and II trials of GM-CSF in patients with idiopathic CD confirmed these observations. Therapy led to improvements in Crohn's disease activity scores, endoscopic disease, draining fistulas, and quality of life. The focus of this application is therefore to study the normal role of endogenous GM-CSF in the mucosa and to establish a mechanistic basis for the therapeutic response to exogenous GM-CSF in CD. Disrupted tolerance to commensal bacteria is central in the pathogenesis of CD. Our general hypothesis is that GM-CSF regulates mucosal immunity and has a therapeutic effect in IBD models and Crohn's disease through effects on tolerogenic dendritic cells. This hypothesis has 3 components, each supported by preliminary data: 1) Mucosal production of GM-CSF contributes to the regulation of immunity by effects on the number and function of dendritic cell subsets. 2) Type I interferon (IFN) producing plasmacytoid dendritic cells (pDC) play a central role in the regulation of normal mucosal tolerance and the therapeutic response to GM-CSF. 3) Administration of exogenous GM-CSF in models of IBD works through increased pDC expression of type I IFN. These will be addressed in 3 aims: (1) Define the role of GM-CSF in the regulation of lamina propria cell populations and mucosal immunity, (2) Define the role of the IFN producing plasmacytoid dendritic cell on the mucosal response to bacteria and bacterial products, (3) Delineate the role of the pDC and IFN in the response to GM-CSF in IBD models. These studies will provide important new insight into the mechanisms used by commensal flora initiate a productive dialog with the host immune system and promote mucosal homeostasis characteristic of healthy individuals. These studies will also improve our understanding of the mechanisms underlying the therapeutic benefit from GM-CSF in recent clinical trials for CD.